Ok Fellas, need HELP.....KX Sluggish but I know it wants to Rock n Roll !!!

[GreenMachine1974]

I just picked up a steal on a 1987 KX 500 $200 !!!!!!!!!!!!!!

The bike seems to be in great shape except for one thing.  It is sluggish towards the Higher RPM's.  Its like the bike can't access the top of its power-band / Governor?  It also sounds like I have a faint Engine noise (rattling).

When I let off the gas / decelertate the bike jerks (it kind of revs up and down beyond my control/slightly).
You know what I mean you get that kind of nnnnnnnn  (slight pause) NNNNNNNNN (pause)  nnnnnnnnnnnnn jerky up and down noise (lol, sorry for the idiotic explanation).

I know the bike's top end has recently been rebuilt from the previous owner who really was an amateur to say the least.  My couch potatoe mechanical diagnosis is I think the problem is one of two things and/or both.  I think I have a bad crank (crankshaft run-out?) and/or Powerband hang-ups (missings parts maybe  or the previous owner installed it wrong ?) 

This bike wants to rock and roll I can tell but it is just being restricted by something ?

Any suggestions would be appreciated.  Also this is a great website I have learned much about my bike on here.  Thanks to all those who help and put up such valuable info (share).

--Matt

[Hillclimb#42]

Congrats on the deal. That issue could be alot of things.  Probably will need more explanations. Engine rattling could be lean condition pinging, or powervalve parts, clutch basket even a worn cylinder. Alot of bikes are supposed to have a new top end, but I wouldn't let that rule out a possible problem there.
 The k5 does have a bit of a run-on when you let off, but can be helped by making sure the slide closes all the way. That means working mechanically and throttle stop screw not holding it open.
 Top Rpms could be jetting, timing, cylinder wear. Lots of discussion for jetting on here. tiimng too far advanced would take some off the top r's. Plenty of other things could be affecting it. What gearing are you running? Could add top with adding a tooth on the countershaft. 500's are often geared down to tame them, but have plenty of bottom end grunt to pull tall gearing.

[GreenMachine1974]

Thanks Man.  I really think this is a powerband issue because its like the valves are not opening up fully because it boggs at higher RPMs.

I also may have a little Engine noise/piston rattle so I think my crank bearings need replacing ?

The hardest thing for me to fix of these 2 problems is the Powervalve assembly.  MY manual is not too clear on disassembly/reassembly of these valves and I think the previous owner put them in wrong, and/or I am missing a part or 2 on these valves.

fun times

[don46]

your quite fortunate it's an 87, the power valves are easy to set, no idler gears to mess with. Pull off your exhaust pipe and the KIPS actuation cover. Look in the exhaust port the two barrel ports should be closed, meaning that the flow from the cylinder is restricted, pull the arm away from the cylinder, the ports should now be open. As far as noise, I always thought the K5 had the death rattle, even when new. The not running on top end, and assuming the piston and rings are good, (dangerous assumption) it sounds like a KIPS issue or maybe a jetting issue, I wouldn't think it would be a timing issue, but could be so you should pop the mag cover and make sure the timing marks are lined up.

[nd4speed]

When you are letting off the gas and decelerating, are you leaving the clutch engaged (not pulling the lever in)?

Also, an engine will rev like crazy and go nowhere fast on top if the clutch is not adjusted and not fully engaging (slipping). May want to check that too. (Clutch basket could be grooved a lot)

Start with the jetting. You can pull the main jet without removing the carb.

Post some pics!

[GreenMachine1974]

Thanks guys.  Almost done rebuilding this badboy.  I took off my engine cover which houses my clutch/kickstarter/and exhaust advance timer.  It appears my exhaust advance timer had slipped off the groove which runs directly to the cylinder to operate the power valves.  I am about 99% sure once I get my gaskets here from Babbits and put this badboy back together I will be roosting down the trail with pride.

Will post some pics soon.

Thanks